/Engine/Source/Runtime/SlateCore/Public/Input/HittestGrid.h
Add an already existing grid that occupy the same space. The grid needs to have an owner, not be this grid and occupy the same space as this grid.
void AddGrid ( const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< const [FHittestGrid](API\Runtime\SlateCore\Input\FHittestGrid) > & OtherGrid )